Summary of Techniques: Troubles attributable to electromagnetic inference (EMI) have increased with the recent functional progress and spread of electric and electronic devices. Electromagnetic noises are classified roughly into conducted noises and radiated noises. Methods of preventing troubles due to conducted noises filter conducted noises by a noise filter. Methods of preventing troubles due to radiated noises use a metal case to shield a space electromagnetically, places a metal sheet between wiring boards or coat the wires of cables with a metal foil. Although effective in electromagnetically shielding circuits and power blocks, these methods are unsuitable for shielding electromagnetic radiation generated by the screens of displays, such as CRTs and PDPs because those methods use opaque means.
The PDP is an assembly of a glass panel provided with data electrodes and a fluorescent layer, and a glass panel provided with transparent electrodes. The PDP generates a large amount of electromagnetic radiation, a large amount of near-infrared radiation and a large amount of heat when operated. Usually, a front panel is disposed in front of the PDP to shield electromagnetic radiation. The front panel must have a shielding function of 30 dB or above in the range of 30 MHz to 1 GHz to shield electromagnetic radiation emitted from the screen of the display. Infrared radiation of wavelengths in the range of 800 to 1,100 nm radiated from the screen of the display must be shielded because infrared radiation makes other devices, such as VTRs, malfunction. The electromagnetic shielding metal mesh (lines) of the electromagnetic shielding sheet must be invisible to facilitate the observation of images displayed on the screen of the display, and the electromagnetic shielding sheet must have proper transparency (visible light transmitting property, visible light transmittance) to make images displayed by the display satisfactorily visible.
Since the PDP is provided with a large screen, such as a 37 in. screen, a 42 in. screen or a greater one, the electromagnetic shielding sheet used in combination with the PDP has thousands of horizontal and vertical lines. The widths of the lines must be within a predetermined width range. If the widths of the lines are distributed in a wide range, black and white spots are formed in a displayed image, which makes a viewer feel a feeling of wrongness.
Prior Art: To ensure improved visibility of displayed images, the front panel is required to have an electromagnetic shielding ability, proper transparency (visible light transmittance), and a uniform, meshed structure capable of electromagnetic shielding.
A front panel having a meshed structure disclosed in JP 5-283889 A has a structure: (base)/(transparent anchor layer)/(electromagnetic shielding layer). The electromagnetic shielding layer has a meshed pattern and is formed by an electroless plating process. A method of forming a metal mesh for an electromagnetic shielding sheet disclosed in JP 09-293989 A uses a photoresist process. An electromagnetic shielding structure disclosed in JP 10-335885 A is formed by laminating a plastic film provided with a copper foil formed in a geometrical pattern by photolithography to a plastic plate. All of those prior art methods mention nothing about the accuracy of the width of the lines of the meshed structure and the influence of the accuracy of the width.
A method of fabricating an electromagnetic shielding sheet disclosed in JP 11-186785 forms lines on a transparent base such that the area of a section of each line in a plane parallel to the transparent base decreases with the distance from the transparent base; that is, the lines has a trapezoidal section in a plane perpendicular to the transparent base. However, the prior art mentions nothing about the distribution of the widths of the lines. Since this method performs overetching or interrupts etching, the control of the accuracy of the width of the lines is very difficult. The lines having widths distributed in a wide range cause irregular image density, have a bad appearance, form black and white spot defects and linear defects in images and deteriorate the visibility of images.